1. Field of Invention
The present invention relates to a process and apparatus for removal of nitrogen oxides (NOx), sulfur oxides (SOx) and mercury from flue gas and other industrial off gas, and is within the technical field of industrial off gas disposal and environmental protection.
2. Related Art
Treatment of off gas like flue gas produced in combustion process and similar chemical process is much important in the field of air pollution control. The types of off gases have common characteristics in that they have high temperature (usually above 100° C.), and contain nitrogen oxides and sulfur oxides of a high concentration as well as a small amount of metal oxide or metal vapor. The majority of the nitrogen oxides is formed by the oxidization of nitrogen in air while a small amount of it is formed by the oxidization of nitrogenous compounds contained in fuel. On a basis of total of the nitrogen oxides NO usually accounts for about 90%, and the balance is mostly NO2. The sulfur oxides, including the predominant of SO2 in addition to a small amount of SO3, primarily originate from oxidization of sulfides in fuel. The metal oxides primarily originate from oxidization of metallic compounds in fuel, and upon cooling, most of the metal oxides are mixed into ashes without entering atmospheric environment, and those having ability to enter atmosphere and causing hazard to environment are the metal vapors with low boiling point, such as zero valent metallic mercury.
For control of SOx (i.e., desulphurization) in flue gas, the wet process, dry process and semi-dry process are used at present. The dry desulphurization process involves injecting the adsorbing and absorbing materials such as the powdered limestone (CaCO3), lime (CaO) or lime hydrate (Ca(OH)2) into a burner or flue after burner to remove SOx. This process is simple, but is low in reaction efficiency, needs a substantial amount of the adsorbing/absorbing materials to be used, has a removal efficiency generally below 70%, and tends to cause scaling on the surface of the heat exchanger, thus frequent rinse on the heat exchanger surface is required. The reacted adsorbing materials increase the concentration of solid particles in the flue gas and impose big challenge to de-dust operation for the flue gas. Moisturizing or injecting SO3 are sometimes required to correspondingly improve the de-dusting efficiency of electrical precipitator. In addition, mixing of the reacted adsorbing material into the fly ashes makes their utilization difficult.
Wet desulphurization process involves absorbing SOx into alkaline liquids through the use of wet scrubber. Typically the alkaline liquids (NaOH or Ca(OH)2) are used as the scrubbing liquid, and the off gas is brought into the scrubber and well contacts with the scrubbing liquid. The process usually provides the removal efficiency of more than 90%, and the produced sulfate and sulfite may be recovered as byproducts. The main disadvantage is that the cost of equipment is higher than that of the dry process and some water is lost into the flue gas; meanwhile, a certain amount of wastewater is produced, which requires additional treatment. In addition to the above two methods, there is a process known as semi-dry scrubbing. In this process, the scrubbing liquid is fed into the scrubber through a nebulizer as tiny droplets, and only a small amount of the scrubbing liquid is required. Main disadvantage of the process is that the pressure drop required for the off gas to pass through the scrubbing device is large, resulting in greater energy consumption; also there are certain restrictions on the temperature of the gas, and the temperature of the gas entering the scrubbing device is generally required to be near or below the saturation temperature of the scrubbed gas.
For NOx removal of the flue gas, the selective catalytic reduction (SCR), the selective non-catalytic reduction (SNCR), oxidization, adsorption/absorption, and the like may be used. The selective catalytic reduction involves injecting ammonia into the flue and letting it to combine with the flue gas, and pass through the catalyst bed, where the ammonia reduced NOx to N2 under the effect of the catalyst. Since this reaction requires a temperature of about 300° C., the catalytic reaction unit has to be mounted in the flue segment between the boiler outlet and the dust precipitator. The catalysts are usually the transition metal oxides, such as V2O5, Fe2O3, CuO and the like. The process is relatively simple in structure and high in removal efficiency, the removal rate can typically reach above 80%. The disadvantages are in that the catalyst is expensive, and the catalyst bed needs to be often washed and replaced owing to that the catalyst bed has to be mounted before the dust precipitator; meanwhile, the ammonia is an unstable compound with pungent odor, and there are potential safety risks in its storage and transportation; in order to ensure good removal efficiency, the ammonia gas generally needs to be added in a properly excessive amount, which will bring the risk of ammonia spilling, and ammonia spilling into the environment would cause a serious impact on atmospheric environment. The selective non-catalytic reduction process involves adding the reducing agent, ammonia or urea, into the burner for reducing NOx at a high temperature of about 1000° C. The process is simple without the use of catalyst, but is low in removal efficiency, which is normally lower than 70%. When injecting the urea, a certain amount of water will inevitably be injected because it is required as the solvent for urea. The evaporation of water will cause a large energy loss. For the adsorbing or absorbing process, the activated carbon or organic metal chelating compounds, such as ferrous ethylene di-amine tetra-acetate (Fe2+EDTA) and tri-ethylene diamine cobalt (Co(En)3)2+, are used to adsorb or absorb NOx. Such methods can often simultaneously remove both SOx and NOx, with high removal efficiency. However, it needs to use a large amount of the adsorbing/absorbing agents which are normally very expensive; in addition, the adsorbing/absorbing agents are difficult to regenerate, and the desorbed NOx or SOx still needs to be disposed after the desorption.
For oxidization method of NOx removal including injection of the oxidizing agent into the flue gas, NO and NO2 are oxidized to NO2 and NO3 which are more soluble in water or the alkaline solution, and thus removed by the absorption in the followed wet scrubbing device, nitrous acid (salts) or nitric acid (salts) are produced respectively. The commonly used oxidizing agents are sodium hypochlorite, hydrogen peroxide, ozone and chlorine dioxide (ClO2) (U.S. Pat. No. 7,628,967 B2), and chlorine gas (U.S. Pat. No. 4,619,608 and U.S. Pat. No. 6,447,740), etc. Sodium hypochlorite and hydrogen peroxide, which are liquid at ambient temperature, are often mixed into the liquid phase in the gas scrubber for use. Ozone, ClO2 and chlorine gas, which are gas at ambient temperature and atmospheric pressure, are usually directly injected into the gas phase for use. The NOx removal by oxidization could provide high removal efficiency as observed in SCR. The oxidants usually have ability to oxidize SO2 and metallic mercury as well, and the process for absorption and removal of NO2 and NO3 requires similar conditions with that for SO2 and SO3, so the NOx removal by oxidization also enables the removal of SO2, SO3 and metallic mercury as well, although the dose of the oxidizing agent, reaction conditions, energy consumption and the like are greatly affected by the type and addition means of the oxidizing agents. For example, ozone, a gas having high oxidization activity, needs to be produced in situ; its production consumes substantial electrical energy. For example again, sodium hypochlorite, chlorine dioxide and chlorine gas contain chlorine, which will cause problems with the wastewater treatment and the utilization of oxidative products, and also the oxidation reaction needs high temperature. The use of hydrogen peroxide does not produce new contaminants and affect the quality of byproducts, however it has relatively low oxidization efficiency at normal temperature, and the addition of a large amount of hydrogen peroxide or elevation of reaction temperature is required to obtain the desirable removal efficiency.
The current methods for removal of mercury (elementary mercury and mercury ion) from flue gas primarily include oxidization and activated carbon adsorption, wherein the oxidization uses halogen gases, such as bromine gas, chlorine gas, iodine molecules and compounds (U.S. Pat. No. 6,878,358 B2 and U.S. Pat. No. 6,447,740), or elementary sulfur and sulfide, including H2S, COS (U.S. Pat. No. 6,972,120 B2) and Na2S4 (EP 0,709,128 A2). These oxidants react with mercury to produce the mercury compounds having high freezing point or good solubility in the scrubbing liquid; the generated mercury compounds are thereby separated from the gaseous phase and go into the solid residue or residual liquid. In U.S. Pat. No. 6,503,470, it was mentioned that sodium hydrosulfide (NaHS) or other sulfur compounds were added for reaction with mercury to produce mercuric sulfide. U.S. Pat. No. 6,447,740 mentioned that the alkali metal iodine was added for reaction with mercury ion to produce HgI2 precipitate. These reaction reagents except chlorine are for the removal of mercury and have less contribution to the removal of other pollution components in the flue gas. Chlorine gas is the common oxidizing agent and is able to treat other gas components. If these methods are solely used for removal of mercury, the cost will be very high. The activated carbon has less adsorptive capacity for mercury comparing with the aforementioned sulfur or halogen compounds, and usually needs to be treated on its surface (such as by impregnating with bromine gas or soaking in bromic acid) in order to obtain the desirable treatment effect.
In U.S. Pat. No. 7,628,967 B2, it is proposed that SOx, NOx and mercury are removed simultaneously from the off gas by multistage scrubbing. Firstly all or part of SOx is absorbed and removed with conventional alkaline liquid; then NOx and remaining SOx are oxidized to higher oxidation state with the injection of the oxidizing agent, and the produced SOx and NOx in the higher oxidation state are absorbed with the alkaline liquid; finally the residual NOx and SOx are removed in a conventional scrubbing step using alkaline liquid. The volatile metallic mercury can be oxidized into ionic mercury in the oxidization section, which can be removed in the subsequent scrubbing device. For the oxidizing agent, hydrogen peroxide, or a mixture of hydrogen peroxide and nitric acid, or a mixture of hydrogen peroxide, nitric acid and chlorous acid, as well as various sodium or potassium salts of chlorinated oxy acids are used. The oxidizing agent is added into the scrubbing liquid for use. Although this process enables simultaneous removal of SOx, NOx and mercury, it is obvious that it is a long process. In addition, the conditions for oxidization reaction are not specified in this patent.
Using hydrogen peroxide as the oxidizing agent for the NOx removal from off gas is more advantageous because it is liquid at ambient temperature, thus convenient for transportation and storage, and would not produce harmful byproducts owing to only hydrogen and oxygen are contained therein. Collins et al (2001) revealed that the most preferred temperature for reaction of hydrogen peroxide with NO was 500° C. At this temperature, H2O2 was decomposed to generate free radical HO., which was further reacted with H2O2 to generate the free radical HO2., a highly active free radical. A desired mole ratio of H2O2 to NO was 1:1, with the removal rate up to about 90%, and the presence of sulfur dioxide had no negative but positive effect on the removal of NOx, which is a feature very favorable for simultaneous SOx and NOx removal. However, the reaction temperature required is very high for this process, and the temperature at the outlet of the burner which is installed with heat exchanger is usually only about 150° C., so the temperature of the flue gas has to be significantly increased to 500° C. to obtain the desirable NOx removal efficiency, which is inevitably costly in the consumption of energy. In addition, the increase of the temperature may have negative effect on the subsequent scrubbing operation, because the gas temperature allowing efficient scrubbing must be below the dew point of the scrubbed gas.
In summary, the known processes for SOx removal of off gas, such as dry scrubbing process, wet scrubbing process or semi-dry scrubbing process, are only effective for SOx removal, could not achieve simultaneous removal of NOx and heavy metals; the known processes for NOx removal of off gas, such as selective catalytic reduction, selective non-catalytic reduction, and adsorbing and absorbing processes, all have the disadvantages such as high cost, undesirable reactions, difficulty in regeneration for the adsorbing/absorbing agents, failure to enable simultaneous removal of SOx and heavy metals at the same time of NOx removal. Although the existing oxidative NOx removal process enables simultaneous removal of SOx and heavy metal mercury, its cost and efficiency largely depend on the method of using the oxidizing agents, in addition to the type of oxidizing agents. Consumption of large amount of electric current is required for generating ozone, and the oxidizing agents containing chlorine, such as various chlorinated oxy acids (perchloric acid, chloric acid, chlorous acid and hypochlorous acid) and sodium or potassium salts thereof, chlorine gas and chlorine dioxide, may introduce the chlorine-containing compounds into the eluent, which will have adverse effect on the corresponding treatment of waste water and byproduct utilization; in addition, the high reaction temperature or overdose addition is usually required for these processes.
Hydrogen peroxide as the oxidizing agent for NOx removal, will neither affect the purity of the eluted products, nor have adverse effect on the corresponding treatment of waste water, but needs a relatively high reaction temperature and thus needs all the flue gas to be heated in order to achieve desirable NOx removal efficiency. It will cause substantial energy consumption, and negative affect on the subsequent scrubbing operation.